1. Field of the Invention
The present invention relates to a process for preparing copper phthalocyanine blue pigments in the alpha form characterized by high yield, good dispersing properties and high stability in the presence of heat or aromatic solvents.
2. Prior Art
The introduction of phthalocyanine pigments in 1935 set new standards of excellence in the pigment consuming industries. They are characterized by their excellent light fastness, intensity, bleed and chemical resistance, extreme stability, and exceptionally high tinting strength. Phthalocyanine pigments are restricted to the blue and green regions of the spectrum. Because of their excellent color values, working properties and low cost in addition to durability, the phthalocyanine blue and green pigments are used extensively.
Copper phthalocyanine blue exists in at least two crystalline modifications, a red shade alpha form, and the more stable green shade beta form. The red shade alpha form is highly desirable but presents a problem in that it must be stabilized in some manner to avoid crystal growth or conversion in the presence of heat and/or aromatic solvents to the green shade beta modification. Such stabilization has been performed in the prior art by the introduction of small amounts of chlorine into the pigment molecule. This has been accomplished in the prior art by for example reacting 3 moles of phthalic anhydride and one mole of a monochlorophthalic acid derivative, e.g., the anhydride, the acid, the monosodium salt. This process has the disadvantage in that a marked reduction in chemical yield results. In U.S. Pat. No. 3,024,247, crude beta copper phthalocyanine is acid pasted to gived a dry alpha pigment. This pigment is reacted dry with chlorine to give a red shade stabilized copper phthalocyanine. While this is a valuable process the product produced is not as easily dispersible as might be desired and there is room for improvement in its usefulness in plastics or in flushing.